1. Field of the Invention
This invention relates to a viscometer for a fluid flowing in a pipe and to a method of determining the viscosity of the fluid.
2. Discussion of the Prior Art
In measurement and automation technology, the viscosity of a fluid flowing in a pipe, particularly of a liquid, is frequently determined by means of meters which, using a vibratory transducer and meter electronics connected thereto, induce internal friction forces in the fluid and derive therefrom a measurement signal representative of the respective viscosity.
Such viscometers are described, for example, in U.S. Pat. No. 4,524,610 in published International Application WO-A 95/16897 and comprise:                a vibratory transducer        with an essentially straight flow tube for conducting the fluid, said flow tube communicating with the pipe and vibrating in operation,        an excitation assembly for vibrating the flow tube,        a central axis of the vibrating flow tube being essentially left in its shape and spatial position, so that the flow tube practically does not leave a static rest position assigned to it,        a sensor arrangement for sensing vibrations of the flow tube and for generating at least one sensor signal representing the vibrations of the flow tube; and        meter electronics which deliver        an excitation current for the excitation assembly and        at least one measured value representing the instantaneous viscosity of the fluid,        the meter electronics        adjusting the excitation current by means of the at least one sensor signal and        generating by means of the excitation current an internal intermediate value representing instantaneous frictions in the fluid, and        the meter electronics determining the viscosity value using the internal intermediate value.        
It has turned out, however, that in spite of viscosity and density being maintained virtually constant, particularly under laboratory conditions, the viscosity value determined by means of the excitation current may exhibit considerable inaccuracies, which may amount to as much as one hundred times the actual viscosity of the fluid.
In U.S. Pat. No. 4,524,610, a possible cause of this problem is indicated, namely the fact that gas bubbles in the fluid may be trapped at the wall of the flow tube. To avoid this problem, it is proposed to install the transducer so that the straight flow tube is in an essentially vertical position, so that the trapping of bubbles is prevented. This, however, is a very specific solution which is only conditionally realizable, particularly in industrial process measurement technology. On the one hand, the pipe into which the transducer is to be inserted would have to be adapted to the latter and not vice versa, which probably cannot be conveyed to the user. On the other hand, the flow tubes may also have a curved shape, so that the problem cannot be solved by adapting the position of installation. It has also turned out that the aforementioned inaccuracies of the measured viscosity value cannot be appreciably reduced even if a vertically installed, straight flow tube is used. Variations in the measured viscosity value of a moving fluid cannot be prevented in this manner, either.